Detection, monitoring and measurement of EVLW (Extravascular Lung Water) are matters of interest for decades. Non-invasive, instant and reliable EVLW measurement method is expected to be superior to existing procedures like blood oxygenation, chest radiography, minimally invasive trans-pulmonary thermodilution for assessment of pulmonary edema.
EVLW was suggested as a predictor of mortality in patients with severe sepsis and Acute Lung Injury (ALI) as a diagnostic tool in detecting early pulmonary edema and in evaluating the effect of ventilator modes during esophagectomy. The measurement has also been proposed to guide fluid therapy in acute respiratory distress syndrome and subarachnoid hemorrhage, and to assess the effect of steroids during cardiac surgery. EVLW was the primary outcome variable in clinical trials to study the efficacy of salbutamol to resolve pulmonary edema in patients with ALI/acute respiratory distress syndrome (the Beta-Agonist Lung Injury Trial) and lung resection.
In patients with acute exacerbations of chronic left heart failure or with acute myocardial infarction, hydrostatic pulmonary edema frequently develops leading to hypoxemia and decreases in lung compliance and ultimately to respiratory failure. Resolution of edema from the alveolar space predicts outcome and is essential for survival in ALI. There are evidences that suggest that a strategy to limit or reduce the amount of extra-vascular lung water (EVLW) from all reduces mortality in and improves the quality of life of patients suffering from EVLW due to preventing treatment. These studies provide strong evidence to support the direct measurement of EVLW in patients in whom a clinical suspicion of pulmonary edema exists, or in those felt to be at risk to develop pulmonary edema and that treatment strategy be specifically designed around attempting to lower elevated EVLW to normal goals.
Eisenberg et al and Mitchell et al in two separate studies (1987 and 1992), first demonstrated that when fluid and hemodynamic management is guided by measured EVLW as opposed to central pressures and usual care, outcome is significantly improved. EVLW fell to a greater extent in patients with ALI and in patients with heart failure and the duration of time spent on the ventilator and in the ICU was less in both studies.
Among the various methods for measurement of EVLW, thermo-dye dilution has been used most frequently. In critically ill patients, fluid management guided by thermo-dye measured EVLW was associated with improved clinical outcome. Hence, EVLW has been suggested to play a role as an independent predictor of the prognosis and course of illness. However, the thermo-dye dilution method is relatively time consuming, cumbersome and expensive. For these reasons, the method has not gained general acceptance. Use of a technique based on injection of a single thermo-indicator that can be detected using an indwelling arterial catheter was an appealing concept. Recent experimental and clinical studies have shown that EVLW assessed by single thermodilution (ST) exhibits good reproducibility and close agreement with the thermo-dye double indicator technique. The invasiveness of the procedure is the main problem that prevents wide clinical use of this procedure. Thermodilution technique requires insertion a catheter in to the central vein and peripheral artery for indicator injection and collection of thermo-dye. Catheterization and puncturing of large blood vessels are complicated procedures often accompanied by various complications and cannot be recommended as routine procedures.
U.S. Pat. No. 5,005,582 discloses a non-invasive method for measuring pulmonary blood flow and lung tissue volume, called airway thermal volume consisting of dynamic registration of respiratory heat losses in ventilatory loading and/or humidity and temperature changes of the inspired gas. Pulmonary blood flow and airway tissue volume are calculated by solving the differential equation for non-steady-state heat and mass exchange between the lungs and the environment. The lungs fraction as natural conditioner of the inspired air, having an inner heat source (pulmonary blood flow) and an outgoing heat stream calculated by measuring the volume ventilation and the temperature and humidity of inspired and expired. air. Alterations of the baseline steady-state condition of lung respiratory heat exchange with the environment by changes in ventilation lead to achievement of a new steady-state condition where the heat stream from the lungs into environment is balanced by the heat stream from the circulation into the lung tissue. The maximal temperature of the expired air is taken as an initial relative value of lung tissue temperature, so that the quantity of maximal expired temperature change between two different steady-state conditions of lung heat exchange is proportional to the pulmonary blood flow, while the rate at which the new steady-state is achieved is proportional to the quantity of tissue mass. A probe for carrying out measurements includes a low-inertial device for temperature and humidity measurements of the expired and inspired air located in the middle of the airstream near the entrance to the upper respiratory tract, combined with a device for gas volumetric measurements and valves dividing in—and outflowing air for minimizing errors in air temperature and humidity measurements